Pulse oximeter

ABSTRACT

There is a need for a technique to compensate for, or eliminate, motion-induced artifacts in patient-attached critical care monitoring instruments. Also, a need exists to extend the accurate operational range of patient-attached pulse oximeters in environments when the patient&#39;s blood oxygen saturation is well below the normal physiologic range, or where there is low blood flow. Accordingly, the invention is directed to improving pulse-oximetry by incorporating additional signals to aid in the triggering of the pulse-oximeter or in analyzing the data received by the pulse oximeter. These approaches include measuring a pulsatile characteristic of the patient at a position close to, or at the pulse-oximetry measurement site, or using pulsatile characteristics that result from contraction of the patient&#39;s heart.

This application is related to Pulse Oximeter With Motion Detection,filed on even date herewith by Victor E. Kimball, which is incorporatedhere by reference.

FIELD OF THE INVENTION

The present invention is directed generally to medical devices and moreparticularly to non-invasive optical sensors for physiologic parameterssuch as blood oxygen saturation content.

BACKGROUND

Optical spectroscopy techniques have been developed for a wide varietyof uses within the medical community. For example, pulse oximetry andcapnography instruments are in widespread use at hospitals, both in thesurgery suites and the post-op ICU's. These technologies havehistorically been based on absorption-based spectroscopy techniques andhave typically been used as trend monitors in critical care environmentswhere it is necessary to quickly determine if a patient's vitalparameters are undergoing large physiologic changes. Given thisoperating environment, it has been acceptable for these devices to havesomewhat relaxed precision and accuracy requirements, given the clinicalneed for real-time point-of-care data for patients in critical caresituations.

Both pulse oximeters and capnography instruments can be labeled asnon-invasive in that neither require penetration of the outer skin ortissue to make a measurement, nor do they require a blood or serumsample from the patient to custom calibrate the instrument to eachindividual patient. These instruments typically have pre-selected globalcalibration coefficients that have been determined from clinical trialresults over a large patient population, and the results representstatistical averages over such variables as patient age, sex, race, andthe like.

There is, however, a growing desire within the medical community fornon-invasive instruments for use in such areas as the emergency room,critical care ICU's, and trauma centers where fast and accurate data areneeded for patients in potentially life threatening situations.Typically, these patients are not anesthetized and motion-inducedartifacts may corrupt data from patient-attached monitoring instruments.Also, patients in shock or acute trauma may have oxygen saturationlevels well below the normal physiologic range, or may suffer fromreduced blood flow.

SUMMARY OF THE INVENTION

Given the situation described above there is a need for a technique tocompensate for, or eliminate, motion-induced artifacts inpatient-attached critical care monitoring instruments. Also, a needexists to extend the accurate operational range of patient-attachedpulse oximeters in environments when the patient's blood oxygensaturation is well below the normal physiologic range, or where there islow blood flow. Consequently, the invention is directed to improvingpulse-oximetry by incorporating additional signals to aid in thetriggering of the pulse-oximeter or in analyzing the data received bythe pulse oximeter. This includes measuring a pulsatile characteristicof the patient at a position close to, or at the pulse-oximetrymeasurement site, or using pulsatile characteristics that result fromcontraction of the patient's heart. One particular embodiment of theinvention is directed to a method of determining hemoglobin oxygensaturation at a measurement site on a patient. The method includesdetecting a first pulsatile characteristic of the patient proximate themeasurement site and making hemoglobin oxygen saturation measurements atthe measurement site. The hemoglobin oxygen saturation measurements madeat the measurement site are then analyzed. One of i) making thehemoglobin oxygen saturation measurements and ii) analyzing thehemoglobin oxygen saturation measurements is performed in response tothe detected first pulsatile characteristic.

Another embodiment of the invention is directed to a system for makingmeasurement of hemoglobin oxygen saturation of a patient at ameasurement site. The system includes means for detecting a pulsatilecharacteristic of the patient proximate the measurement site and meansfor making hemoglobin oxygen saturation measurements at the measurementsite. The system also includes means for analyzing the hemoglobin oxygensaturation measurements made at the measurement site. One of i) themeans for making the hemoglobin oxygen saturation measurements and ii)the means for analyzing the hemoglobin oxygen saturation measurementsresponds to the detected pulsatile characteristic.

Another embodiment of the invention is directed to an apparatus formeasuring oxygen saturation of hemoglobin at a measurement site on apatient. The apparatus includes a controller having a first input toreceive a first pulsatile input signal based on a first pulsatilepatient characteristic measured proximate the measurement site, a firstoutput to control making hemoglobin oxygen saturation measurements and asecond input to receive signals related to the measurements of oxygensaturation of hemoglobin made at the measurement site. The controllerincludes a processor that i) controls making the hemoglobin oxygensaturation measurements or ii) analyzes the received signals in responseto the detected first pulsatile characteristic.

Another embodiment of the invention is directed to a sensor unit formaking measurements of hemoglobin oxygen saturation on a patient. Thesensor unit includes a body attachable to the patient. The body has oneor more optical ports for delivering light to the patient at first andsecond wavelengths for measuring hemoglobin oxygen saturation. At leasta portion of a detector is mounted on the body to measure a pulsatilecharacteristic of the patient.

Another embodiment of the invention is directed to a method ofdetermining hemoglobin oxygen saturation at a measurement site on apatient. The method includes measuring a first pulsatile characteristicarising from contraction of the patient's heart and illuminating themeasurement site with light at two different wavelengths. Light at thetwo different wavelengths is detected at the measurement site to producedetection signals. The detection signals are analyzed to determinehemoglobin oxygen saturation levels at the measurement site. One of i)illuminating the measurement site and ii) analyzing the detected lightis performed in response to the measured first pulsatile characteristic.

Another embodiment of the invention is directed to a system fordetermining hemoglobin oxygen saturation at a measurement site on apatient. The system includes means for measuring a first pulsatilecharacteristic arising from contraction of the patient's heart and meansfor illuminating the measurement site with light at two differentwavelengths. The system also includes means for detecting the light atthe two different wavelengths at the measurement site to producedetection signals and means for analyzing the detection signals todetermine hemoglobin oxygen saturation levels at the measurement site.One of i) the means for illuminating the measurement site and ii) themeans for analyzing the detected light performs in response to themeasured first pulsatile characteristic.

Another embodiment of the invention is directed to a system formeasuring oxygen saturation of hemoglobin at a measurement site on apatient. The invention includes a controller having a first input toreceive a first pulsatile measurement signal, indicative of a firstpulsatile characteristic resulting from contraction of the patient'sheart, from the patient, a first output to control making hemoglobinoxygen saturation measurements and a second input to receive signalsrelated to the measurements of oxygen saturation of hemoglobin made atthe measurement site. The controller also includes a processor that i)controls making the hemoglobin oxygen saturation measurements or ii)analyzes the received signals in response to the received firstpulsatile measurement signal.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 illustrates a non-invasive pulse-oximetry unit attached to apatient's fingertip for the determination of blood oxygen saturationlevels;

FIG. 2 shows a block schematic diagram of an enhanced pulse-oximetryunit according to an embodiment of the present invention;

FIGS. 3A and 3B schematically illustrates cross-sectional views ofpulse-oximetry sensors, that include a laser Doppler velocimetrydetector, for attachment to a patient, according to embodiments of thepresent invention;

FIG. 4 shows a block schematic diagram of an enhanced pulse-oximetryunit according to another embodiment of the present invention;

FIGS. 5A and 5B schematically illustrate cross-sectional views ofpulse-oximetry sensors, that include impedance sensors, for attachmentto a patient, according to other embodiments of the present invention;

FIG. 6 schematically illustrates a non-invasive pulse-oximetry unit thatincorporates motion detection, according to an embodiment of the presentinvention;

FIG. 7 schematically shows the temporal dependence of pulse-oximetersignals, impedance signals and laser Doppler velocimetry signals;

FIG. 8 schematically illustrates a pulse-oximeter sensor head thatincludes a detector for detecting patient motion, according to anembodiment of the present invention;

FIG. 9 schematically illustrates contents of data buffers used inanalyzing pulse-oximetry data;

FIG. 10 schematically illustrates an embodiment of a pulse-oximetrysystem that includes inputs for two pulsatile characteristics, inaddition to the pulse-oximetry measurements, according to an embodimentof the present invention; and

FIG. 11 schematically shows timing diagrams for motion detection and thegeneration of a motion detection signal according to an embodiment ofthe present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to medical devices and is believedto be particularly useful for non-invasive optical physiologic sensors.

Generally, the present invention relates to a method of measurement thataugments the existing non-invasive pulse oximetry methodologies.Advanced algorithms have been developed recently for improvingpulseoximetry measurements, based on only measurements of hemoglobinoxygen saturation. The present invention is directed to the use ofadditional input signals, that may be used along with the newalgorithms, to improve the quality of the pulse-oximetry measurement. Inaddition to pulse oximetry, other optical measurement techniques such asthe non-invasive measurement of blood glucose can benefit from thepresent invention.

Pulse oximetry utilizes the pulsatile nature of blood flow tosynchronize the optical measurement of oxygenated and reduced(de-oxygenated) hemoglobin. Typical commercially available pulseoximeters have two light emitting diodes (LEDs) operating at twodifferent wavelengths, λ1 and λ2, one in the red region near 660nanometers (nm) where the difference between the absorbance ofoxygenated (HbO₂) and reduced (Hb) hemoglobin is the greatest, and asecond LED in the near infrared (NIR) region. The NIR LED typicallyoperates either near 805 nm, where the absorbance of HbO₂ and Hb aresubstantially the same (the isosbestic wavelength), or near 940 nm forincreased sensitivity. An optical detector is typically mounted in ahousing unit with the LEDs to detect the optical energy eithertransmitted through, or reflected from, the patient's tissue. Pulseoximeter sensing heads may be clipped to the patient's fingertip or thepatient may insert a fingertip into a sensor housing unit. In otherapproaches, the pulse oximeter may be attached to the patient's head,for example the patient's ear lobe, or forehead. Use of the head forpulse oximetry measurements is advantageous because the motion of thehead may be less than that of a hand, and in situations where blood flowis reduced, the flow of blood to the head may be reduced less than theflow of blood to an extremity, such as a finger. An increasinglyimportant application for pulse-oximetry is fetal monitoring ofhemoglobin oxygen saturation during childbirth. In this application, thepulse-oximeter is placed on the neonate's head.

Another location on the head where pulse oximetry measurements may bemade is the buccal region (cheek), which is suitable for both reflectionand transmission measurements. Making the pulse-oximetry measurement onthe inside of the cheek may reduce the effects of skin color (melanin)on the measurement, particularly if the measurement is made inreflection. If the measurement is made in transmission, then the lightpasses through the epidermis only once, whereas reflective ortransmissive measurements made on the ear lobe or finger require passageof the light through the epidermis twice, and so even transmission pulseoximetry measurements have the advantage of reduced melanininterference.

In most pulse-oximetry systems, an average over multiple blood pulses istaken to increase the signal-to-noise or signal-to-background ratio.Different approaches have been developed to synchronize the measurementsto the peak and valley of the blood pulse(s) in the time domain as eachindividual blood pulse transits the region near the sensing LEDs anddetector. In one approach, the ECG-R wave is used as a trigger toinitiate a timing sequence to capture the peaks and valleys ofconsecutive optical pulses. If, however, the patient were to move theirarm or finger during the measurement time interval, this movement, oracceleration of the arm or finger, may induce a change in the velocityof the blood entering the sensing region. This may, in turn, cause thefixed timing sequence to miss the peak of the pulsatile wave and thuscorrupt the measurement process. Also, motion of the patient may resultin moving the sensor relative to the skin, which distorts themeasurement. Lastly, the signal to noise of the measurement decreaseswhen the blood flow is reduced, and so it is difficult to obtain goodpulse oximetry measurements under conditions of low blood flow. Thefollowing description is directed to proposed solutions to reducemotion-induced, and low flow-induced error mechanisms in pulse-oximetrysystems.

A schematic representation of a pulse oximeter system 100 attached to apatient's fingertip 102 is presented in FIG. 1. The patient's hand 104is typically not immobilized and is free to move throughout themeasurement time interval. The sensor head 106 containing the LEDs anddetector is attached to the patient's fingertip 102. The sensing head106 is coupled to the main signal processing unit 108 by an interconnect110. The interconnect 110 may be electrical or optical. The system mayinclude a data display 112 for displaying the measured level ofhemoglobin oxygen saturation, and any other information.

Some of the approaches to reducing motion-induced error includemonitoring a pulsatile characteristic of the patient, such as anelectroencephalographic (ECG) signal, heart beat, blood pressure, bloodflow, impedance, and the like, and using this measured pulsatilecharacteristic either to trigger the pulse-oximeter measurements, or inthe analysis of the pulse-oximeter measurements. The pulsatilecharacteristic of the patient may be measured using any suitableapproach. For example, where the pulsatile characteristic is the heartbeat, a sonometer may be used, such as a stethoscope. In other examples,blood pressure may be measured using a blood pressure cuff, blood flowvelocity may be measured using laser Doppler velocimetry (LDV) andimpedance may be measured using an impedance measurement system. Theexamples discussed below are particularly directed to the use of LDV andimpedance measurements, but it will be appreciated that these arepresented as examples, and that the invention is not limited to the useof LDV and impedance measurements, and may be used with other pulsatilecharacteristics. A particular advantage of LDV is that an LDV signalhaving a good signal to noise ratio may still be obtained even underconditions of low blood flow. While non-invasive measurements ofpulsatile characteristics have been listed, where the measurement ismade using a device that does not penetrate the patient's skin, itshould be appreciated that invasive measurements of pulsatilecharacteristics may also be used.

It should also be noted that, although the level of hemoglobin oxygensaturation does vary over a period of a heartbeat with a pulsingvariation, hemoglobin oxygen saturation is not considered to be apulsatile characteristic, for the purposes of the present discussion.

Some pulsatile characteristics, for example, blood pressure, blood flow,and impedance, are quite different in nature from the ECG waveform. TheECG waveform is related to the electrical impulses that drive the heartbeat, and originates in the central nervous system. In contrast, otherpulsatile characteristics such as blood pressure, heart beat, bloodflow, changing impedance and the like arise from the contraction of theheart.

Furthermore, pulse-oximetry measurements are not limited to being madeon the patient's finger, but may also be made elsewhere on the patient'sbody. Some other suitable regions for making pulse-oximetry measurementsinclude, but are not limited to the toe, the ear lobe, the buccal regionand the sublingual region.

In one particular embodiment of the invention that uses LDV, the sensinghead 106 may include elements for making make laser Doppler velocity(LDV) measurements of blood flow in substantially the same region wherethe pulse oximetry measurements are made. Electro-optic signalprocessing techniques to implement LDV measurements are described inU.S. Pat. No. 5,587,785, titled “Laser Doppler Velocimeter”, byinventors Saturo Kato et. al., and also in U.S. Pat. No. 5,502,558,titled “Laser Doppler Velocimeter”, by inventors James H. Menders et.al., both of which are incorporated herein by reference.

In one embodiment of an LDV-enhanced pulse oximetry system, the activeelectro-optics used for making LDV measurements, for example laser,detector, discrete optical components, and the like, may be housed inthe pulse-oximeter main signal processing unit 108. In such a case, afiber optic waveguide may be incorporated in the interconnect cable 110to deliver and receive optical energy from the sensing head 106. Thisconfiguration is attractive from a commercialization viewpoint in thatit does not add considerable cost to the re-usable sensor unit formed bythe sensor head 106 and the interconnect cable 110. In thisconfiguration, with the LDV sensing region/volume substantially the sameas the pulse-ox sensing region/volume, the intrinsic time delay betweenLDV signal and the pulse-ox signal is close to zero. Accordingly, theLDV signal may be used to derive an SaO₂ measurement that has improvedsignal to noise ratio. For example, the LDV signal may be usedalgorithmically to anticipate and/or compensate for patientmotion-induced artifacts.

A schematic representation of one embodiment of an LDV-enhanced pulseoximeter system 200 attached to a patient's fingertip 202 is presentedin FIG. 2. The LDV electro-optics unit 204 may be incorporated in themain housing unit 206 of the LDV-enhanced pulse oximeter 200. The outputof the LDV laser unit 208 is coupled into a fiber-optic waveguide 210,which delivers the laser energy, at a wavelength, λ_(LDV), to thepulse-ox sensing head 212 and returns the Doppler-shifted signal, at awavelength, λ+Δλ_(LDV), to the LDV detector unit 214. The output of theLDV detector unit 214 is coupled to an analyzer unit 216 which mayperform operations such as discrete Fourier transform analysis todetermine the magnitude of the blood velocity. The analyzer unit 216 mayalso detect changes in velocity, in other words a concomitantacceleration component, nested in the LDV signal. The output of theanalyzer unit 216 is coupled to a processor unit 218 which may generatetiming signals to be used by the pulse-ox module 220.

The unit 206 may also output the results of the pulse-oximetrymeasurements and/or the pulsatile measurements directly, on a display230. The displayed information may be presented digitally, for exampleas a series of numerical values, or may be presented graphically, forexample as a function of time, or may be presented in some other manner.In addition, the system 200 may have some other type of output 232 fortransferring data, including both pulse-oximetry data and LDV data. Forexample, the output 232 may be a parallel or serial data port forcommunicating with other computer equipment.

The pulse-ox module 220 may contain the electronics to generate the LEDexcitation signals, via the LED driver circuitry 222, and thehardware/algorithm necessary to process the returning LED signals andcalculate the blood oxygen saturation, performed in the SaO₂ analyzerunit 224. The LED drive signals are delivered by electrical interconnect226 to the sensor unit 212. The returning pulse-ox signal is carried byelectrical interconnect 228 to the SaO₂ analyzer unit 224.

The pulse-ox signals received by the SaO₂ analyzer 224 typically show,during a heart-beat cycle, a peak absorption, corresponding to the peakflow of blood, resulting from constriction of the left ventricle, duringthe heart-beat cycle. At other times during the heart beat cycle, theabsorption falls back to a minimum value before the next cycle startsagain. The processor unit 218 may, based on the input received from theLDV unit 204, generate timing or gating pulses to synchronously detectthe peaks and valleys of repetitive pulse-ox signals, which aretypically used in the algorithm to calculate blood oxygen saturation.

While the processor 218 is illustrated to be separate from the LDV unit204 and the pulse-oximeter module 220, it will be appreciated that thisseparation is only to show separate functions. In practice, a singlemicroprocessor unit, or multiple microprocessors, may be used to processthe data obtained via LDV and via the pulse-oximetry measurements, andto perform the function of the processor 218. This also holds for theadditional embodiments of pulse-oximetry systems illustrated below.

An expanded view of the sensing region depicting one embodiment of thesensor head unit 302 on the patient's finger 304 is schematicallyillustrated in FIG. 3A. The sensor head unit 302 may contain the firstlight source 306 and the second light source 308 and a photodetector310, arranged so that light 316 and 318 from the light sources 306 and308 illuminates the patient's finger 304. The light sources 306 and 308,which are typically LEDs, may straddle the detector 310 so as to besubstantially equidistant from the detector 310. The light 316 at λ1from the first source 306 passes through the patient's finger 304 to thedetector 310. The light 318 at λ2 from the second light source 308 alsopasses through the patient's finger 304 to the detector 310, albeitalong a different optical path. The time-dependent variation in theamount of light reaching the detector 310 at λ1 and λ2 is used todetermine the level of oxygen saturation of the hemoglobin. It should beappreciated that hemoglobin oxygen saturation measurements may also bemade with light at more than two wavelengths.

A fiber optic waveguide 312 for receiving the Doppler shifted light forthe LDV measurement may be positioned proximate the detector 310 suchthat the LDV measurement is made in substantially the same location ofthe patient as the hemoglobin oxygen saturation measurements. In oneembodiment, the fiber optic waveguide 312 is oriented so that the lightincident on the finger from the waveguide 312 is incident on the surfaceof the finger 304 at an angle other than perpendicular, although thefiber optic waveguide 312 may also be oriented so that its axis isperpendicular to the finger 304. Furthermore, the light from the fiberoptic waveguide 312 may be coupled directly into the patient, or throughone or more optical elements, such as a lens, prism and the like. Thephotodetector for the LDV measurement may be positioned in the LDVmodule 204, in which case the fiber optic waveguide 312 forms a portionof the detector for the measurement of the pulsatile characteristic, theLDV signal.

It will be appreciated that other arrangements may also be used. Forexample, the pulse oximeter may, instead of locating the light sources306 and 308 on the sensor head unit 304, include the light sources 306and 308 in the pulse-ox module 220 itself, and may direct the light atλ1 and λ2 to the patient via one or two fiber optic waveguides.Furthermore, the detector 310 need not be placed on the surface of thepatient's skin, but may be optically coupled, for example via an opticalfiber, to the patient's skin.

In another embodiment, one or more of the active electro-opticalcomponents of the LDV unit 204 may be positioned at the sensor head unit302. For example, the LDV laser unit 208 and/or the LDV detector unit214 may be positioned on the sensor head unit 302. In anotherembodiment, the LDV laser unit 208 may be used to replace one of thelight sources 306 and 308, so that the pulse oximetry measurements aremade using light at wavelength λ_(LDV) and at either λ₁ or λ₂. In such acase, λ_(LDV) is typically selected to be at a wavelength useful forpulse oximetry measurements.

The LDV measurements need not be made in the same portion of thepatient's tissue as the pulse-oximetry measurements, and the LDVmeasurements may also be made at a different position on the patient.For example, where the pulse-oximetry measurements are made at the apatient's digit, such as a finger or toe, the pulse oximetrymeasurements may be made close to the that digit, for example at anotherdigit on the same hand or foot, on the lower limb to which the digit isattached, or to the limb to which the digit is attached. The closer theLDV measurement is made to the site of the pulse-oximetry measurement,then the smaller is the time delay between the measured pulsatilecharacteristic and the features of the pulse-oximetry measurement. Itwill be appreciated, however, that LDV measurements may be made at anyposition of the patient's body. These different variations justdescribed may also be used in other embodiments discussed below.

Another embodiment of a sensor head 350 that includes LDV measurement isschematically illustrated in FIG. 3B. This sensor head 350 is adaptedfor pulse-oximeter measurements made in transmission through thepatient's tissue. Such measurements may be made at many locations of thepatient including, but not limited to, a digit, the ear lobe and thecheek. Measurements of physiologic characteristics of a patient that aremade at the cheek and other epithelial tissues are further discussed inU.S. patent application Ser. No. 10/195, 005, entitled “Method ForMeasuring A Physiologic Parameter Using A Preferred Site,” incorporatedherein by reference.

In this particular embodiment, the sensor head 350 is split into twoparts that are placed on either side of a tissue flap 352, such as adigit, ear lobe, cheek or the like. The first part 350 a of the sensorhead 350 includes two light sources 356 and 358, operating at differentwavelengths, λ1 and λ2. The second part 350 b of the sensor head 350includes a light detector 360 that detects light from the two sources356 and 358. A fiber optic waveguide 362 is positioned on the secondpart 350 b to direct LDV probe light 364 at a wavelength Of ALDV intothe tissue flap 352, and to receive the frequency shifted light from thesampled tissue for the LDV measurement.

In another embodiment, the detector 360 may be used to detect theDoppler-scattered LDV signal light. One way of distinguishing the LDVlight from the pulse-oximetry light while using a single detector 360 isto modulate the LDV light and pulse-oximetry light at differentfrequencies. In addition, the fiber optic waveguide 362 may be placed onthe first part 350 a, wihle the detector 360 is on the second part 350b.

A schematic representation of one embodiment of an impedancemeasurement-enhanced (Z-enhanced) pulse oximeter unit 400 is illustratedin FIG. 4. An impedance measurement of a patient is typically made bypassing a current between two active electrodes and measuring theresulting potential difference between two passive electrodes. Themeasured impedance depends on the electrical conductivity of the tissueand fluid through which the current passes. The pulse of the bloodstreamcontributes to a pulsatile change in the amount and type of fluid in thecurrent path, which is detected as a pulsatile change in the measuredimpedance.

The unit 400 includes an impedance measuring unit 404 and apulse-oximetry module 420. The impedance measuring unit 404 may beincorporated in the main housing unit 406 of the Z-enhanced pulseoximeter 400. The impedance measuring unit 404 includes a signalgenerator 408 and a detector 414. Impedance measurements of a patientare typically made using a system of at least four electrodes, with thetwo outer electrodes being actively driven with a signal and the twoinner electrodes passively receiving a signal related to the activesignal and the impedance of the patient. Accordingly, the output of thesignal generator unit 408 is coupled to an interconnect 410, whichdelivers the input signal, signal_(in), to the sensing head 412, todrive the active electrodes. The interconnect 410 also returns theimpedance signal, signal_(out), from the passive electrodes to theimpedance detector unit 414. The electrical interconnect 410 may includemultiple electrical leads connecting the signal generator 408 to thepulse-ox sensing head 412, and the input and output signals from thesensing head may be on different electrical leads. An example of thistype of configuration of impedance measurement is described in U.S. Pat.No. 5,178,154, titled “Impedance Cardiograph and Method of OperationUtilizing Peak Aligned Ensemble Averaging”, which is incorporated hereinby reference.

The output of the impedance detector unit 414 is coupled to theimpedance analyzer 416 which may perform operations such as firstderivative tests to determine and align repetitive peak values. Theoutput 417 of the analyzer unit 416 may be coupled to a processor unit418 which may then generate timing signals to be used for triggering thepulse-ox module 420. The analyzer unit 416 may also output the impedancemeasurements directly, so that the operator is informed of theimpedance. The impedance may be presented digitally, for example as aseries of numerical values, or may be presented graphically, for exampleas a function of time, or may be presented in some other manner.

The pulse-ox module 420 typically contains the electronics to generatethe LED excitation signals, via the LED driver circuitry 422, and thehardware/algorithm necessary to process the returning LED signals andcalculate the blood oxygen saturation, performed in the SaO₂ analyzerunit 424. The LED drive signals are delivered by electrical interconnect426 to the sensor unit 412, and the returning pulse-ox signal is carriedby electrical interconnect 428 to the SaO₂ analyzer unit 424. Theprocessor unit 418 may generate timing or gating pulses to synchronouslydetect the peaks and valleys of repetitive pulse-ox signals,corresponding to positions of maximum and minimum light absorption bythe blood, and which are typically used in the algorithm to calculateblood oxygen saturation. The processor unit 418 may use any appropriateapproach for analyzing the incoming impedance data. For example, theimpedance data may be subject to a fast Fourier transform, a discreteFourier transform, or to other transform or filtering methods. Severalapproaches are available for using the analyzed impedance signal toimprove the signal to noise of the results of the pulse oximetrymeasurements. Some of these approaches are taught by analogy in U.S.Pat. No. 5,178,154. The unit 400 may include a display 430 fordisplaying pulse-oximetry results to the operator. The display 230 mayalso display the measured pulsatile characteristic, in this case theimpedance. The information may be presented digitally, for example as aseries of numerical values, or may be presented graphically, for exampleas a function of time, or may be presented in some other manner. Inaddition, the system 400 may have some other type of output 432 fortransferring data, including both pulse-oximetry data and the pulsatiledata. For example, the output 232 may be a parallel or serial data portfor communicating with other computer equipment. An expanded view of thesensing region depicting one particular embodiment of the sensor headunit 502 on the surface of the patient's finger 504 is schematicallyillustrated in FIG. 5. The sensor head unit 502 may contain the firstlight source 506 and the second light source 508, which may be LEDs. Thelight sources 506 and 508 may straddle the detector 510 so as to besubstantially equidistant from the detector 510.

In this particular embodiment, the impedance measurement technique iscarried out using a pair of active electrodes 512 and 514, and a pair ofpassive electrodes 516 and 518. The electrodes 512 and 514 contact thepatient's finger 504 to provide a current stimulus to the finger 504.The current flowing between electrodes 512 and 514 develops a voltagedrop which can be measured between electrodes 516 and 518, the voltagebeing a product of the current and the tissue and/or blood impedance.The voltage measured between electrodes 516 and 518 may then be detectedby the detector 414 and analyzed in the analyzer 416.

In the illustrated configuration, the impedance measurement may be madein substantially the same physical location as the pulse-ox measurement.Also, in this configuration the impedance signal typically contains apulsatile component synchronous with the arrival of blood pulses. Insuch a case, the measured impedance signal may be used as a trigger toinitiate pulse-ox measurements, to ensure that the pulse-ox measurementsare made synchronously with the flow of blood in the patient.

Another embodiment of a sensor head 550 that includes impedancemeasurement is schematically illustrated in FIG. 5B. This sensor head550 is adapted for pulse-oximeter measurements made in transmissionthrough the patient's tissue. Such measurements may be made at manylocations of the patient including, but not limited to, a digit, the earlobe and the cheek. Measurements of physiologic characteristics of apatient that are made at the cheek and other epithelial tissues arefurther discussed in U.S. patent application Ser. No. 10/195, 005,entitled “Method For Measuring A Physiologic Parameter Using A PreferredSite.”

In this particular embodiment, the sensor head 550 is split into twoparts that are placed on either side of a tissue flap 552, such as adigit, ear lobe, cheek or the like. The first part 550 a of the sensorhead 550 includes two light sources 556 and 558, operating at differentwavelengths, λ1 and λ2. The second part 550 b of the sensor head 550includes a light detector 560 that detects light from the two sources556 and 558. The sensor head 550 also includes first and second activeelectrodes 562 and 564 for applying a current to the tissue flap 552,and two passive electrodes 566 and 568 for measuring a potentialdifference that arises due to the current passing between the acitveelectrodes 562 and 564.

In this particular embodiment, the electrodes are positoined such thatthe current path 570 between the active electrodes 562 and 564 passesthrough the region sampled by the pulse-oximetry light sources 556 and558. Furthermore, a line 572 drawn between the passive electrodes 566and 568 crosses current path 570.

A timing diagram 700 is presented in FIG. 7, showing the relative timingbetween an ECG signal 701, a pulse-oximeter signal 702, an impedancesignal 704, and an LDV signal 706. In this case, the impedance and LDVsignals are assumed to have been made proximate the pulse-oximetermeasurement site, for example as shown in FIGS. 3 and 5, so that thereis little relative delay between the impedance or LDV measurements andthe change in light absorption resulting from the pulse of blood passingthe measurement site.

In this configuration, the minima of the pulse-oximetry signal 708,corresponding to increased optical absorption in the blood may occur atsubstantially the same time (t_(min)) as the minima 710 of the impedancesignal 704, where the minima in both signals may represent the arrivalof a blood pulse. Likewise, the minimum in 708 the pulse-oximeter signal702 substantially coincides with the maximum 716 in the LDV signal 706,corresponding to maximum blood flow velocity.

Also, the maxima 712 of the pulse-oximetry signal 702 may occur atsubstantially the same time (t_(max)) as the maxima 714 of the impedancesignal 704, and the minima 718 of the LDV signal 706. Accordingly, theimpedance signal 704 and/or the LDV signal 706 may be used as a triggerfor collecting pulse-oximetry data. Where the LDV or impedancemeasurement are made proximate the measurement site for thepulse-oximetry measurements, the timing between the impedance and/or LDVmeasurement and the pulse-oximetry signal does not significantly change,even if the patient moves. Accordingly, the light sources of thepulse-oximeter may be triggered to pulse at around t_(min) and t_(max),in which case the noise of the ensemble average should be reduced.

In another approach, illustrated in FIG. 9, pulse-oximetry data arereceived as a train of sampled data representing a sampling of thepulse-oximeter signal 702. The data are stored in a buffer, with anassociated time stamp. The pulse-oximetry data are represented asPOD_(i), where the subscript “i” is an integer, while the time stampdata are represented by t_(i). Data from a pulsatile measurement, suchas an LDV measurement or impedance measurement, represented as PMD_(j),are also stored, along with associated time stamps, represented bytt_(j). A selected value of PMD_(j), PMD_(jmax), that represents apredetermined value for which it is known that the associated bloodabsorption value is a maximum, is selected. The value of POD_(imax),whose time stamp, t_(imax) is closest to tt_(jmax), may then be selectedas the value of PMD that represents the maximum value of the bloodabsorption. Similarly, the time tt_(jmin), that represents apredetermined time for which it is known that the associated bloodabsoprtion is at a minimum may then be selected, and the value ofPOD_(imin), whose time stamp, t_(imin), is closest to tt_(jmin), may beselected as the value of PMD that represents the minimum value of bloodabsorption. The hemoglobin oxygen saturation may then be calculatedusing PMD_(imax) and PMD_(imin).

As indicated above, impedance and LDV signals are examples of pulsatilepatient characteristics that may be used to enhance pulse-oximetrymeasurements. Pulsatile characteristics that may be used fall into twobroad categories. One is pulsatile characteristics that result fromcontraction of the heart, such as blood pressure and acoustic heart beatsignal and the like. This category also includes impedance and LDVcharacteristics. The other category is pulsatile characteristics that donot result from contraction of the heart. An example of this type ofcharacteristics is an ECG signal.

The device used for measuring the pulsatile characteristic may be placedproximate the site where the pulse-oximetry measurement is made. Here,the term “proximate” is intended to cover positions not only near themeasurement site, but also at the measurement site. Advantages providedby proximate measurement of the pulsatile characteristic include thatthe relative timing between the pulse-oximetry measurement and thepulsatile characteristic is less sensitive to patient motion.Accordingly, the improvement in pulse oximetry signal to noise thatresults from including the pulsatile characteristic is also lesssensitive to patient motion. Additionally, the combination of differenttypes of sources, detectors and other components into a single sensorhead may lead to overall reduction in the cost of manufacturing animproved pulse-oximeter system.

In other embodiments, the pulsatile characteristic may be measured atsome location removed from the pulse-oximetry measurement site. Forexample, impedance measurements made on the patient's torso may be usedin conjunction with pulse-oximetry measurements made on the patient'sdigit. It will be appreciated that the further removed the pulsatilemeasurement is from the pulse-oximetry measurement site, the larger thedelay may be between the timing of the pulsatile characteristic and thetiming of the change in optical absorption detected in thepulse-oximetry measurements. Measurements made at one position on thepatient may be less sensitive to motion artifacts than at others.

A problem with using an ECG signal as a pulsatile characteristic is thatECG monitoring typically involves the placement of several electrodeswidely spaced across the patient's torso. The measurement of pulsatilecharacteristics arising from contraction of the heart, on the otherhand, may be measured using a single sensor, or a set of closely spacedsensors on a single housing attached to the patient. Furthermore, thepatient may have a heart condition in which the stimulus to the heart isfaulty. Use of a pulsatile characteristic arising from contraction ofthe heart avoids problems arising from a faulty stimulus, or from aheart that responds to the stimulus in an abnormal way. Since the goalof the pulse oximetry measurement is to measure a quantity affected bythe flow of blood around the body it is, therefore, advantageous to useas additional signals other inputs that are directly related to the flowof blood around the body.

In another embodiment of the invention, the pulse-oximetry unit mayinclude the input of two or more pulsatile signals to aid in thegeneration of the measurement of hemoglobin oxygen saturation. Oneembodiment of such a system 1000 is schematically illustrated in FIG.10. For example, the unit 1000 may include two inputs 1002 and 1004 forpulsatile signals that result from contraction of the heart, in additionto an input 1006 for receiving pulse-oximetry data. In such a case, thefirst input signal may be detected and/or analyzed by a firstdetector/analyzer 1012 and the second input signal may be detectedand/or analyzed by a second detector/analyzer 1014. The pulse oximetrymeasurements received at the third input 1006 may be detected and/oranalyzed in the pulse oximeter unit 1016. The pulsatile signals appliedto the two inputs 1002 and 1004 may be associated with different typesof pulsatile characteristic, or may be associated with the same type ofcharacteristic. Furthermore, the signals applied to the two inputs 1002and 1004 may be made at the same position on the patient, or atdifferent positions. To illustrate, the signals applied to the inputs1002 and 1004 may both be LDV signals, measured at the same position onthe patient or at different places.

The processor 1018, in addition to performing the functions discussedabove with regard to triggering the pulse oximetry measurement andanalyzing the pulse oximetry data, may also perform various checks orconfirm operations to determine that the pulsatile data received areuseful For example, the processor 1018 may compare the data received atthe two inputs 1002 and 1004 to determine whether there is an expectedcorrespondence between the two measurements of pulsatilecharacteristics. The processor 1018 may then determine whether to useeither of the measured pulsatile signals to trigger the pulse oximeterunit 1016 or to otherwise aid in the analysis of the pulse oximetermeasurements. The processor 1018 may also determine a signal to noiselevel for each of the measured pulsatile signals and use that measuredpulsatile signal having the more useful signal to noise level fortriggering the pulse oximeter measurements or otherwise aiding in theanalysis of the pulse oximeter measurements.

The unit 1000 may output measured pulse-oximetry and pulsatile resultson a display (not shown) or may transmit measurement data to otherequipment on some other type of output (not shown).

One of the inputs 1002 and 1004 may also include an ECG measurement orother pulsatile characteristic that is not generated by contraction ofthe heart. It may be useful to compare a pulsatile characteristicgenerated by contraction of the heart with, for example, an ECG signalas a control to ensure that the pulsatile characteristic generated bycontraction of the heart corresponds in some manner to the ECG signal.For example, one test may be to ensure that the pulsatile characteristicresulting from contraction of the heart occurs at a rate equal to therate of the ECG signal. An event where the rates change or where therelative phasing between the two signals changes may be indicative of achange in status of the patient, and may even be sufficient reason forsetting an alarm. In addition, a change in the relative nature of thetwo pulsatile signals may be used to indicate a failed measurement ofone of the two pulsatile signals.

Another factor that may affect the usefulness of the pulse-oximetrymeasurement is the motion of the patient while the measurement is takingplace. In particular, motion of the patient, for example that part ofthe patient where the pulse-oximetry measurements are being made, mayresult in the change of timing between any trigger for the pulse-oxmeasurements and the absorption extrema in the pulse-oximetrymeasurements. For example, if a virtual trigger is used to set thewindow for pulse-oximetry measurements that relates to a single heartbeat, movement of the patient may result in the absorption maximum inthe pulse-oximetry measurement moving around within that window. Thisrelative time shift of the pulse-oximetry measurement as a result ofpatient movement is particularly of concern when the motion is large,random, and/or violent, for example when the patient is in shock, isshivering, or is having a seizure.

One particular example of a pulse-oximeter system 600 that may be usefulin reducing the problems associated with patient motion is schematicallyrepresented in FIG. 6. In this embodiment, a pulse-oximeter sensor unit606 is attached to the patient's fingertip 602. The patient's hand 604is not immobilized and is free to move while the pulse-oximetrymeasurements are being made. The sensor head 606 containing the LED'sand detector is attached to the patient's fingertip 602. The sensinghead 606 is attached to the main signal processing unit 608 by aninterconnect 610.

In one embodiment of the present invention, the sensing head 606 mayincorporate a motion sensor, such as a linear accelerometer, which maysense erratic motion, periodic motion (such as tapping), or even violentmotion of the sensing head 606. Such motions may interfere with thenormal operation of the pulse-oximetry system. Where the detected motionlies above a particular threshold, for example in the case of extreme orviolent motion of the sensing head 606, the signal from the motionsensor may be used to generate a blanking command in the main signalprocessing unit 608 to disregard data collected during those incidents.Conversely, during periods where the sensing head 606 is stationary, thesignal from the motion sensor may be used as a gating command tovalidate raw data as acceptable for downstream processing. In anotherapproach, the main signal processing unit may be programmed to not takepulse-oximetry data while the motion sensor detects motion that exceedsa particular threshold.

In one embodiment of an accelerometer, the motion sensor is a smallpiezo-electric device commercially available from multiple vendors,e.g., National Instruments Inc., Austin, Tex.

In another embodiment of the present invention, the motion sensor may bemounted on the patient's adjacent fingertip(s) 612 and may be coupled tothe main signal processing unit 608 via electrical interconnect 614. Inanother embodiment, the motion sensor 618 may be mounted elsewhere onthe patient, for example on the patient's forearm 616. Theseparately-mounted sensor 618 may be coupled to the main signalprocessing unit 608 via an electrical interconnect 620. The unit display622 may indicate that the patient's movement has exceeded a certainthreshold and, for example, may be sufficient to adversely affect thepulse-oximetry measurement.

FIG. 11 shows two curves showing the relative timing of a blankingsignal and a motion signal. Curve 1102 is an example of the output fromthe motion sensor, showing relative calm before time t1 and after t2. Inthe period between t1 and t2, however, the patient is moving, as isreflected in the motion sensor output. The pulse-oximeter system maygenerate a blanking signal 1104 related to the detected motion. Forexample, the blanking signal may turn on when the motion is detected tobe above a certain threshold, and may stay on if the motion exceeds thatthreshold within a certain period of time following previous excursionbeyond that threshold. The blanking signal 1104 may also be setaccording to different motion criteria.

The blanking signal 1104 may be used for any process used in analyzingthe data that is related to the patient's motion. For example, theblanking data may be used to tag data taken during periods of motiondeemed to be too excessive to make accurate pulse-oximetry readings, sothose data taken during periods of excessive motion may be ignored whenproducing a pulse-oximetry result. The motion signal 1102 may alsootherwise aid in making a pulse-oximetry measurement. For example, theunit may analyze the motion signal 1102 to determine the frequency orfrequencies of the patient's motion and then, when analyzing thepulse-oximetry data, may filter out those artifacts having a frequencyor frequencies corresponding to the motion frequencies.

An embodiment of a sensor head 800 attachable to the patient, forexample at the patient's finger 802, is schematically illustrated inFIG. 8. The sensor head 800 has a body 804 that includes first andsecond light sources 806 and 808 for directing light into the patient attwo different wavelengths and a photodetector 810 for detecting thelight signals. The light sources 806 and 808 and the photodetector 810may be used for making measurements of the hemoglobin oxygenconcentration. The sensor head 800 also as a motion sensor 812 that maybe integrated with the body 804. The motion sensor 812 may be coupled tothe pulse-oximeter controller to provide information regarding patientmotion. The sensor head 800 may also include at least part of sensor 814for measuring a pulsatile characteristic of the patient. In theillustrated example, the pulsatile sensor 814 includes a fiber 816 forreceiving an LDV signal from the patient. The fiber 816 may also directthe LDV probe light to the patient.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

We claim:
 1. A method of determining hemoglobin oxygen saturation at ameasurement site on a patient, comprising: noninvasively detecting afirst pulsatile characteristic of the patient proximate the measurementsite; making hemoglobin oxygen saturation measurements at themeasurement site; and analyzing the hemoglobin oxygen saturationmeasurements made at the measurement site; wherein at least one of i)making the hemoglobin oxygen saturation measurements and ii) analyzingthe hemoglobin oxygen saturation measurements and iii) any combinationthereof is performed in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and wherein making the hemoglobin saturationmeasurements includes triggering the hemoglobin oxygen saturationmeasurements at a predetermined time relative to the first pulsatilecharacteristic.
 2. A method of determining hemoglobin oxygen saturationat a measurement site on a patient, comprising: noninvasively detectinga first pulsatile characteristic of the patient proximate themeasurement site; making hemoglobin oxygen saturation measurements atthe measurement site; and analyzing the hemoglobin oxygen saturationmeasurements made at the measurement site; wherein at least one of i)making the hemoglobin oxygen saturation measurements and ii) analyzingthe hemoglobin oxygen saturation measurements and iii) any combinationthereof is performed in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and wherein detecting the first pulsatilecharacteristic includes making a laser Doppler velocimetry (LDV)measurement proximate the measurement site.
 3. A method as recited inclaim 2, further comprising making the LDV measurement at themeasurement site.
 4. A method of determining hemoglobin oxygensaturation at a measurement site on a patient, comprising: noninvasivelydetecting a first pulsatile characteristic of the patient proximate themeasurement site; making hemoglobin oxygen saturation measurements atthe measurement site; and analyzing the hemoglobin oxygen saturationmeasurements made at the measurement site wherein at least one of i)making the hemoglobin oxygen saturation measurements and ii) analyzingthe hemoglobin oxygen saturation measurements and iii) any combinationthereof is performed in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and wherein detecting the first pulsatilecharacteristic includes measuring impedance of the patient proximate themeasurement site.
 5. A method as recited in claim 4, further comprisingmeasuring the impedance of the patient at the measurement site.
 6. Amethod of determining hemoglobin oxygen saturation at a measurement siteon a patient, comprising: noninvasively detecting a first pulsatilecharacteristic of the patient proximate the measurement site; makinghemoglobin oxygen saturation measurements at the measurement site; andanalyzing the hemoglobin oxygen saturation measurements made at themeasurement site; wherein at least one of i) making the hemoglobinoxygen saturation measurements and ii) analyzing the hemoglobin oxygensaturation measurements and iii) any combination thereof is performed inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable; andwherein the measurement site is at a patient's digit and the firstpulsatile characteristic is detected on the limb to which the digit isattached.
 7. A method as recited in claim 6, wherein the first pulsatilecharacteristic is detected on the lower limb portion of the limb towhich the digit is attached.
 8. A method of determining hemoglobinoxygen saturation at a measurement site on a patient, comprising:noninvasively detecting a first pulsatile characteristic of the patientproximate the measurement site: making hemoglobin oxygen saturationmeasurements at the measurement site: and analyzing the hemoglobinoxygen saturation measurements made at the measurement site: wherein atleast one of i) making the hemoglobin oxygen saturation measurements andii) analyzing the hemoglobin oxygen saturation measurements and iii) anycombination thereof is performed in response to the detected firstpulsatile characteristic regardless of whether the first pulsatilecharacteristic is stable or unstable; and wherein the measurement siteis at a patient's digit and the pulsatile characteristic is detected onone of the patient's digits.
 9. A method of determining hemoglobinoxygen saturation at a measurement site on a patient, comprising:noninvasively detecting a first pulsatile characteristic of the patientproximate the measurement site; making hemoglobin oxygen saturationmeasurements at the measurement site; and analyzing the hemoglobinoxygen saturation measurements made at the measurement site; wherein atleast one of i) making the hemoglobin oxygen saturation measurements andii) analyzing the hemoglobin oxygen saturation measurements and iii) anycombination thereof is performed in response to the detected firstpulsatile characteristic regardless of whether the first pulsatilecharacteristic is stable or unstable; and wherein the measurement siteis at a patient's digit and the pulsatile characteristic is detected onthe patient's digit.
 10. A method of determining hemoglobin oxygensaturation at a measurement site on a patient, comprising: noninvasivelydetecting a first pulsatile characteristic of the patient proximate themeasurement site; making hemoglobin oxygen saturation measurements atthe measurement site; and analyzing the hemoglobin oxygen saturationmeasurements made at the measurement site; wherein at least one of i)making the hemoglobin oxygen saturation measurements and ii) analyzingthe hemoglobin oxygen saturation measurements and iii) any combinationthereof is performed in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and wherein the measurement site is at thepatient's buccal region.
 11. A method of determining hemoglobin oxygensaturation at a measurement site on a patient, comprising: noninvasivelydetecting a first pulsatile characteristic of the patient proximate themeasurement site; making hemoglobin oxygen saturation measurements atthe measurement site; and analyzing the hemoglobin oxygen saturationmeasurements made at the measurement site; wherein at least one of i)making the hemoglobin oxygen saturation measurements and ii) analyzingthe hemoglobin oxygen saturation measurements and iii) any combinationthereof is performed in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and further comprising detecting motion of thepatient; and wherein one of i) making the hemoglobin oxygen saturationmeasurements and ii) analyzing the hemoglobin oxygen saturationmeasurements is performed in response to the detected motion.
 12. Amethod of determining hemoglobin ox en saturation at a measurement siteon a patient comprising: noninvasively detecting a first pulsatilecharacteristic of the patient proximate the measurement site; makinghemoglobin oxygen saturation measurements at the measurement site; andanalyzing the hemoglobin oxygen saturation measurements made at themeasurement site; wherein at feast one of i) making the hemoglobinoxygen saturation measurements and ii) analyzing the hemoglobin oxygensaturation measurements and iii) any combination thereof is performed inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable; andfurther comprising detecting a second pulsatile characteristic of thepatient, wherein the one of i) making the hemoglobin oxygen saturationmeasurements and ii) analyzing the hemoglobin oxygen saturationmeasurements is performed in response to the detected second pulsatilecharacteristic.
 13. An apparatus for measuring oxygen saturation ofhemoglobin at a measurement site on a patient, comprising: a controllerhaving a first input to receive a first pulsatile input signal based ona first pulsatile patient characteristic measured noninvasively andproximate the measurement site, a first output to control makinghemoglobin oxygen saturation measurements and a second input to receivesignals related to the measurements of oxygen saturation of hemoglobinmade at the measurement site, and having a processor that at least oneof i) controls making the hemoglobin oxygen saturation measurements andii) analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable;wherein the controller triggers a signal at the first output at apredetermined time relative to the pulsatile input signal.
 14. Anapparatus for measuring oxygen saturation of hemoglobin at a measurementsite on a patient, comprising: a controller having a first input toreceive a first Pulsatile input signal based on a first pulsatilepatient characteristic measured noninvasively and proximate themeasurement site, a first output to control making hemoglobin oxygensaturation measurements and a second input to receive signals related tothe measurements of oxygen saturation of hemoglobin made at themeasurement site, and having a processor that at least one of i)controls making the hemoglobin oxygen saturation measurements and ii)analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable;wherein the controller further includes a laser Doppler velocimetry(LDV) unit and the first input receives LDV data from the patient. 15.An apparatus as recited in claim 14, wherein the controller LDV unitincludes a light source for generating light used in generating the LDVdata received from the patient.
 16. An apparatus for measuring oxygensaturation of hemoglobin at a measurement site on a patient, comprising:a controller having a first input to receive a first pulsatile inputsignal based on a first pulsatile patient characteristic measurednoninvasively and proximate the measurement site, a first output tocontrol making hemoglobin oxygen saturation measurements and a secondinput to receive signals related to the measurements of oxygensaturation of hemoglobin made at the measurement site, and having aprocessor that at least one of i) controls making the hemoglobin oxygensaturation measurements and ii) analyzes the received signals and iii)any combination thereof in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and wherein the controller further includes animpedance measuring unit and the first input receives impedancemeasurement data from the patient.
 17. An apparatus as recited in claim16, wherein the controller further includes a second output from theimpedance measuring unit to carry signals for generating impedancemeasurements.
 18. An apparatus for measuring oxygen saturation ofhemoglobin at a measurement site on a patient, comprising: a controllerhaving a first input to receive a first pulsatile input signal based ona first pulsatile patient characteristic measured noninvasively andproximate the measurement site, a first output to control makinghemoglobin oxygen saturation measurements and a second input to receivesignals related to the measurements of oxygen saturation of hemoglobinmade at the measurement site, and having a processor that at least oneof i) controls making the hemoglobin oxygen saturation measurements andii) analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable; and asensor head coupled to the controller and couplable to the patient fortaking measurements of hemoglobin oxygen saturation; wherein the sensorhead also includes at least one optical port for transmitting an LDVprobe signal to the patient and receiving an LDV return signal from thepatient.
 19. An apparatus for measuring oxygen saturation of hemoglobinat a measurement site on a patient, comprising: a controller having afirst input to receive a first pulsatile input signal based on a firstpulsatile patient characteristic measured noninvasively and proximatethe measurement site, a first output to control making hemoglobin oxygensaturation measurements and a second input to receive signals related tothe measurements of oxygen saturation of hemoglobin made at themeasurement site, and having a processor that at least one of i)controls making the hemoglobin oxygen saturation measurements and ii)analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable; and asensor head coupled to the controller and couplable to the patient fortaking measurements of hemoglobin oxygen saturation: wherein the sensorhead delivers light to the patient at first and second wavelengths formaking measurements of hemoglobin oxygen saturation, the firstwavelength being different from the second wavelength; and wherein thesensor head delivers light to the patient at a third wavelength formaking an LDV measurement on the patient.
 20. An apparatus for measuringoxygen saturation of hemoglobin at a measurement site on a patient,comprising: a controller having a first input to receive a firstpulsatile input signal based on a first pulsatile patient characteristicmeasured noninvasively and proximate the measurement site, a firstoutput to control making hemoglobin oxygen saturation measurements and asecond input to receive signals related to the measurements of oxygensaturation of hemoglobin made at the measurement site, and having aprocessor that at feast one of i) controls making the hemoglobin oxygensaturation measurements and ii) analyzes the received signals and iii)any combination thereof in response to the detected first pulsatilecharacteristic regardless of whether the first pulsatile characteristicis stable or unstable; and a sensor head coupled to the controller andcouplable to the patient for taking measurements of hemoglobin oxygensaturation; wherein the sensor head delivers light to the patient atfirst and second wavelengths for making measurements of hemoglobinoxygen saturation, the first wavelength being different from the secondwavelength; and wherein the sensor head delivers light to the patient atone of the first and second wavelengths for making an LDV measurement onthe patient.
 21. An apparatus for measuring oxygen saturation ofhemoglobin at a measurement site on a patient, comprising: a controllerhaving a first input to receive a first pulsatile input signal based ona first pulsatile patient characteristic measured noninvasively andproximate the measurement site, a first output to control makinghemoglobin oxygen saturation measurements and a second input to receivesignals related to the measurements of oxygen saturation of hemoglobinmade at the measurement site, and having a processor that at least oneof i) controls making the hemoglobin oxygen saturation measurements andii) analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable;further comprising a third input to receive motion detection informationof the patient, the processor that i) controls making the hemoglobinoxygen saturation measurements or ii) analyzes the received signals inresponse to the motion detection information.
 22. An apparatus asrecited in claim 21, further comprising a motion detector coupled to thethird input.
 23. An apparatus for measuring oxygen saturation ofhemoglobin at a measurement site on a patient, comprising: a controllerhaving a first input to receive a first pulsatile input signal based ona first pulsatile patient characteristic measured noninvasively andproximate the measurement site, a first output to control makinghemoglobin oxygen saturation measurements and a second input to receivesignals related to the measurements of oxygen saturation of hemoglobinmade at the measurement site, and having a processor that at least oneof i) controls making the hemoglobin oxygen saturation measurements andii) analyzes the received signals and iii) any combination thereof inresponse to the detected first pulsatile characteristic regardless ofwhether the first pulsatile characteristic is stable or unstable;further comprising a fourth input to receive a second pulsatile inputsignal based on a second pulsatile characteristic, the processor i)controlling making the hemoglobin oxygen saturation measurements or ii)analyzing the received signals in response to the detected secondpulsatile characteristic.
 24. A sensor unit for making measurements ofhemoglobin oxygen saturation on a patient, comprising: a body attachableto the patient, the body having one or more noninvasive optical portsfor noninvasively delivering light to the patient at at least first andsecond wavelengths for noninvasively measuring hemoglobin oxygensaturation; and at least a portion of a noninvasive detector mounted onthe body to measure noninvasively a pulsatile characteristic of thepatient; and wherein the at least a portion of the detector includes anoptical port for receiving an optical laser Doppler velocimetry (LDV)signal from the patient.
 25. A sensor unit as recited in 24, wherein theoptical port includes an optical fiber.
 26. A sensor unit as recited 24,for making measurements of hemoglobin oxygen saturation on a patient,comprising: a body attachable to the patient, the body having one ormore noninvasive optical ports for noninvasively delivering light to thepatient at at least first and second wavelengths for noninvasivelymeasuring hemoglobin oxygen saturation; and at least a portion of anoninvasive detector mounted on the body to measure noninvasively apulsatile characteristic of the patient; further comprising a motionsensor attached to the body.
 27. A method of determining hemoglobinoxygen saturation at a measurement site on a patient, comprising:measuring a first pulsatile characteristic arising from flow of blood inthe patient; illuminating the measurement site with light at at leasttwo different wavelengths; detecting the light at the at least twodifferent wavelengths at the measurement Site to produce detectionsignals; and analyzing the detection signals to determine hemoglobinoxygen saturation levels at the measurement site; wherein at least oneof i) illuminating the measurement site; ii) detecting the light; iii)analyzing the detected light; and iv) any combination of the foregoingis performed in response to the measured first pulsatile characteristicregardless of whether the first pulsatile characteristic is stable orunstable.
 28. A method as recited in claim 27, wherein illuminating themeasurement site includes triggering the one or more light sources at apredetermined time relative to the first pulsatile characteristic.
 29. Amethod as recited in claim 27, wherein analyzing the detected lightincludes selecting hemoglobin oxygen saturation measurement data inresponse to the first pulsatile characteristic.
 30. A method as recitedin claim 27, further comprising measuring a second pulsatilecharacteristic of the patient and wherein at least one of i)illuminating the measurement site and ii) analyzing the detected lightis performed in response to the measured second pulsatilecharacteristic.
 31. A method as recited in claim 30, wherein the secondpulsatile characteristic is a pulsatile characteristic arising fromcontraction of the heart.
 32. A method as recited in claim 30, whereinthe second pulsatile characteristic is an electrocardiographic signal.33. A method as recited in claim 27, wherein detecting the lightincludes measuring light absorption at a time of substantially maximumoptical blood absorption and measuring light absorption at a time ofsubstantially minimum optical blood absorption.
 34. A method as recitedin claim 33, wherein the times of maximum absorption and minimumabsorption correspond to predetermined times relative to a feature ofthe first pulsatile characteristic.
 35. A method as recited in claim 27,further comprising detecting motion of at least part of the patient toproduct motion detection signals, and wherein analyzing the detectionsignals includes analyzing the detection signals in response to themotion detection signals.
 36. A method as recited in claim 27, whereinmeasuring the first puisatile characteristic includes measuring bloodflow velocity.
 37. A method as recited in claim 27, wherein measuringthe first puisatile characteristic includes measuring impedance.
 38. Amethod as recited in claim 27, wherein measuring the first pulsatilecharacteristic includes measuring blood pressure.
 39. A method asrecited in claim 27, wherein measuring the first puisatilecharacteristic includes measuring an acoustic signal.
 40. A method asrecited in claim 27, further comprising detecting motion of at leastpart of the patient and wherein analyzing the detection signals includesanalyzing the detection signals includes selecting detection signalsbased on whether the detection signals were obtained during a period ofpatient motion.
 41. A method as recited in claim 27, further comprisingoutputting results related to the first pulsatile characteristic arisingfrom contraction of the patient's heart.
 42. A method as recited inclaim 27, further comprising displaying results related to the firstpulsatile characteristic.
 43. A system for determining hemoglobin oxygensaturation at a measurement site on a patient, comprising: means formeasuring a first pulsatile characteristic arising from flow of blood inthe patient; means for illuminating the measurement site with light attwo different wavelengths; means for detecting the light at the twodifferent wavelengths at the measurement site to produce detectionsignals; and means for analyzing the detection signals to determinehemoglobin oxygen saturation levels at the measurement site; wherein atleast one of i) the means for illuminating the measurement site; ii) themeans for detecting the light; iii) the means for analyzing the detectedlight; and iv) any combination of the foregoing performs in response tothe measured first pulsatile characteristic regardless of whether thefirst pulsatile characteristic is stable or unstable.
 44. A system formeasuring oxygen saturation of hemoglobin at a measurement site on apatient, comprising: a controller having a first input to receive afirst pulsatile measurement signal indicative of a first pulsatilecharacteristic resulting from flow of blood in the patient from thepatient, a first output to control making hemoglobin oxygen saturationmeasurements and a second input to receive signals related to themeasurements of oxygen saturation of hemoglobin made at the measurementsite, and having a processor that at least one of i) controls making thehemoglobin oxygen saturation measurements or ii) analyzes the receivedsignals in response to the received first pulsatile measurement signalregardless of whether the first pulsatile characteristic is stable orunstable.
 45. A system as recited in claim 44, wherein the firstpulsatile characteristic is blood flow velocity.
 46. A system as recitedin claim 45, wherein the controller includes a laser Doppler velocimetry(LDV) module, having an output for directing an LDV optical probe signalat the patient and being coupled to receive the first pulsatilemeasurement signal.
 47. A system as recited in claim 44, wherein thefirst pulsatile characteristic is impedance.
 48. A system as recited inclaim 47, wherein the controller includes an impedance module, having anoutput for directing a probe current to the patient and being coupled toreceive the first pulsatile measurement signal.
 49. A system as recitedin claim 44, wherein the first pulsatile characteristic is an acousticsignal.
 50. A system as recited in claim 49, wherein the controllerincludes an acoustic signal module coupled to receive the firstpulsatile measurement signal.
 51. A system as recited in claim 44,wherein the first pulsatile characteristic is blood pressure.
 52. Asystem as recited in claim 51, wherein the controller includes an bloodpressure measuring module couple to receive the first pulsatilemeasurement signal.
 53. A system as recited in claim 44, wherein thecontroller triggers a signal at the first output at a predetermined timerelative to the first pulsatile measurement signal.
 54. A system asrecited in claim 44, wherein the controller selects data obtained fromthe second input in response to the first pulsatile measurement signal.55. A system as recited in claim 44, wherein the controller furtherincludes a third input to receive a second pulsatile measurement signalfrom the patient related to a second pulsatile characteristic of thepatient.
 56. A system as recited in claim 55, wherein the secondpulsatile characteristic is a pulsatile characteristic resulting fromcontraction of the patients heart.
 57. A system as recited in claim 55,wherein the second pulsatile characteristic is an electrocardiographicsignal.
 58. A system as recited in claim 44, wherein the controllerfurther includes a pulse-oximeter unit coupled to the first output andthe second input.
 59. A system as recited in claim 58, wherein thepulse-oximeter unit receives optical absorption data from themeasurement site at a time of substantially maximum absorption and at atime of substantially minimum absorption.
 60. A system as recited inclaim 59, wherein the times of maximum a sorption and minimum absorptioncorrespond to predetermined times relative to a feature of the firstpulsatile measurement signal.
 61. A system as recited in claim 44,further comprising a sensor head coupled to the controller andattachable to the patient for taking measurements of hemoglobin oxygensaturation.
 62. A system as recited in claim 61, wherein the sensor headalso includes at least part of a sensor for detecting the firstpulsatile characteristic.
 63. A system as recited in claim 62, whereinthe sensor head delivers light to the patient at first and secondwavelengths for making measurements of hemoglobin oxygen saturation, thefirst wavelength being different from the second wavelength.
 64. Asystem as recited in claim 62, wherein the sensor head delivers right tothe patient at a third wavelength for making an LDV measurement on thepatient.
 65. A system as recited in claim 62, wherein the sensor headdelivers light to the patient at one of the first and second wavelengthsfor making an LDV measurement on the patient.
 66. A system as recited inclaim 62, wherein the sensor head includes an impedance sensor formeasuring impedance of the patient.
 67. A system as recited in claim 62,wherein the sensor head includes a pressure monitor for measuring bloodpressure of the patient.
 68. A system as recited in claim wherein thesensor head includes an acoustic sensor for measuring an acousticsignal.
 69. A system as recited in claim 44, wherein the controllerfurther includes a fourth input to receive patient motion information.70. A system as recited in claim 69, wherein the controller furtheranalyzes the received signals in response to the patient motioninformation received via the fourth input.
 71. A system as recited inclaim 69, further comprising a motion detector coupled to the fourthinput, the motion detector being attachable to the patient.
 72. A systemas recited in claim 71, further comprising a sensor unit attachable tothe patient for receiving at least one of a) the first pulsatilemeasurement signal from the patient and b) optical blood absorptiondata, the motion sensor being integrated to the sensor unit.
 73. Asystem as recited in claim 44, wherein the controller further includes adata output to output data related to the first pulsatile characteristicresulting from contraction of the patient's heart.
 74. A system asrecited in claim 44, wherein the controller further includes a displayto display data related to the first pulsatile characteristic resultingfrom contraction of the patient's heart.